This invention relates generally to aviation structures and, more specifically, to direction of airflow over airframe surfaces.
The scientific principles of aviation are highly dependent upon the shapes of airfoils and attendant assemblies in an on-rushing stream of air. As the air flows past the outer skin of a given shape, deviations from planar surfaces might have profound effects, as in the application of the Bernoulli effect on the upper surface of a wing. As the rate of the airflow varies, the optimum shape to cause a given effect also varies.
As another example, optimum airflow for an engine varies with speed and altitude as well as a host of other variables. As a jet engine passes through air, the volume of air that passes through the jet engines varies with its speed. It is advantageous to control the volume of air with an iris at the intake duct. When too much air is allowed into the duct, excess air spills back causing drag. When too little air is allowed into the duct, the engine cannot run at its most efficient performance levels.
A general solution to this problem is a compromise such as selecting an average geometry of the intake duct. While not optimum, an average geometry most closely approximates an optimum geometry over the greatest portion of the operating spectrum. For some specialized applications, the cost of a variable intake duct is justified for achieving an optimum geometry. As an example, SR-71 Blackbird varies intake geometry to control air flow with an elaborate hydraulic system with an iris in the center of the intake duct.
Over time, the hydraulic system of the SR-71 has proven to be both costly and very heavy. Deployment of hydraulic actuators within the laminar airflow of a duct has proven as well to be inefficient. Additionally, previous designs have experienced problems with the support of seals and aerodynamic leaks.
Some experimental uses have shown that many of the same benefits of intake air flow control can be achieved with an iris that is actuated not from the center but from the sides by variation of the planar skin of the planar duct. By systematically bulging the inner sides of the intake duct, the diameter of the intake duct varies according to need. To date, however, the actuators necessary for such bulging have been heavy and difficult to place in the airframe due to the need for solid spots on the airframe to push against the wall of the intake duct. No such system has entered into production.
Thus, there is an unmet need in the art for effecting variations in a planar skin without aerodynamic leaks, weight, or complexity.
The present invention provides an apparatus and method for varying a wall skin to alter airflow over the skin. The apparatus has a first and second end-plates, a plurality of flexible rods are arranged substantially parallel to substantially define a plane, each flexible rod has a mid-point and first and second ends secured to the first and second end-plates respectively. Each mounting structure is slidingly attached to the flexible rods between the mid-point and each end-plate, proximate to the end. An elastomer envelops the rods to form a flexible skin. A plurality of shape memory alloy rods are arranged parallel to and define a plane and having each end secured to an end-plate, the plurality of shape memory alloy rods being contractible when heated such that upon contraction the plurality of shape memory alloy rods will buckle the plurality of flexible rods and the flexible skin to alter the airflow.
When arranged around the inside walls of a inlet duct to a jet engine, the flexible skin forms an effective iris for controlling airflow into the intake of the jet engine. By controlling the signal applied to the Shape Memory Alloy (SMA) rods, a sensor system can optimize the airflow to the jet assuring most efficient propulsion. The elastomer used in conjunction with the flexible rods provides compliance the metallic structures lack while providing good load carrying capability. The shape memory alloy rods present an uncomplicated lightweight and easily controllable actuator system when compared to the hydraulic equipment.